Many devices exist for sensing engine speed. Each provides a signal having a frequency or a period indicative of that speed.
Some of these devices provide an indication of a predetermined speed or when that predetermined speed has been exceeded. A device may provide a speed signal indicating when a predetermined speed limit has been exceeded by an increasing speed passing through the speed limit and to a point above the speed limit or a decreasing speed passing through the speed limit in a downward direction to a point below the speed limit.
Many of these devices use a combination of timing circuits employing resistances and capacitances or other timing means.
An example of a resistance capacitance timing circuit speed switch is shown in U.S. Pat. No. 4,086,888.
These devices may be analogue or digital, especially where analogue techniques can be adapted to digital practices.
However, these prior art devices have been limited in temperature stability and in noise isolation capability which are important if not critical in the environment of the internal combustion engine and other rotating machinery.